Gas panel display systems of the type to which this invention relates are well known, but it will be helpful to review the features and terminology that particularly apply to this invention. Gas panel displays have two glass plates that are spaced apart by a seal to contain an ionizable medium. A set of spaced horizontally extending "row" conductors are located on one glass plate and a set of vertically extending "column" conductors are located on the other plate. When a suitable voltage is applied between one of the row conductors and a column conductor ionization occurs between the spaced crossover point of the two conductors and light is emitted. The crossover points are called cells and a display pattern is formed by ionizing selected cells. The operation of initially ionizing a cell is called writing. The operation of removing the wall charges from a previously written cell is called erasing. A write pulse is formed by the difference between a pulse applied to the selected horizontal conductor and a pulse applied to the selected vertical conductor. These voltage amplitudes are called half-select voltages, and cells along a row or column conductor that receive a half-select voltage are said to be half-selected. The write half-select voltage must be high enough to produce a full-select voltage to ionize the selected cell, but it must be low enough not to produce an unintended write operation on any other half-selected cells. After the write operation, periodic output of the cell is sustained by an alternating polarity voltage that is called a sustain voltage. The sustain voltage is applied simultaneously to all cells and the previously written cells ionize and accumulate charge for the next sustain operation, but the previously erased cells remain unionized.
In the gas panel displays transistor switches are provided for each one of the row wires and for each one of the column wires. The switches are controlled by addressing counters which connect the individual row and column wires to selectively receive voltages for write and erase operations.
The very accurate resolution of the differential capacitive stylus and associated circuitry of this invention uses the conventional gas panel addressing circuits to provide an indication of the position of the stylus on the display. Hand-held sensors utilized with cathode ray tube displays are well known. Some of these are referred to as light pens and are manually held to the display and receive a light signal form the adjacent illuminated area of the cathode ray tube display. Associated components of the display operate in response to the light signal to identify the coordinate address of the pen. Light pen system used with cathode ray tube systems tend to have slow response in following the movement of the light pen from one illuminated area to another. Sensor systems reacting to the electron beam of the cathode ray tube rather than the illuminated spot are also known. There are many application programs that use a signal telling that a light pen or electron beam sensor has been held to the display or that it has been held to a particular location on the display. An electron beam position sensor for a cathode ray tube display is disclosed in U.S. Pat. No. 3,413,515. Light pen sensors have also been adapted for gas panel displays. A light pen system for a gas panel is disclosed in U.S. Pat. No. 3,851,327.